In a nitride semiconductor light emitting device, hydrogen is undesirably included in a p-type nitride semiconductor layer upon the growth of the p-type nitride semiconductor layer. At this time, the p-type nitride semiconductor layer has the properties of an insulator, but not the properties of a semiconductor due to hydrogen. It is thus required that an additional activation annealing process for removing hydrogen be performed after the p-type nitride semiconductor layer has been grown.
U.S. Pat. No. 5,306,662 discloses a method for eliminating hydrogen through annealing at a temperature of 400° C. or more after a p-type nitride semiconductor layer is grown. U.S. Pat. No. 5,247,533 discloses a method for forming a p-type nitride semiconductor layer by irradiation of a n electron beam.
Therefore, such a conventional process is complicated as well as an underlying active layer can be thermally damaged due to the annealing process, which results in a high possibility of degrading the performance of the device.
Meanwhile, U.S. Pat. No. 6,043,140 proposes a method for fundamentally prohibiting the introduction of hydrogen upon the growth of p-type GaN using a nitrogen precursor and a nitrogen carrier from which hydrogen is not generated. It is, however, not easy to obtain satisfactory surface morphology that can be applied to a light emitting device through this method.
FIG. 1 is a cross-sectional view illustrating the structure of a conventional nitride semiconductor light emitting device. A method for fabricating the conventional nitride semiconductor light emitting device will be hereinafter described in brief. Referring to FIG. 1, the nitride semiconductor light emitting device includes a buffer layer 11, a lower contact layer 12 composed of a n-type nitride semiconductor, an active layer 13 composed of a nitride semiconductor, and an upper semiconductor layer 14 composed of a p-type nitride semiconductor, all of which are sequentially grown on an insulating substrate 10.
Thereafter, an activation annealing process is performed in which hydrogen contained in the upper contact layer 14 is removed at a high temperature of 400° C. or more. A transparent electrode layer 15 is then formed on the upper contact layer 14 that is brought into ohmic-contact with the transparent electrode layer 15. The upper contact layer 14 and the active layer 13 are mesa-etched to expose the lower contact layer 12. An n-type ohmic metal electrode layer 16 is formed on the lower contact layer 12 and a bonding pad 17 is then formed on the transparent electrode layer 15. Finally, a protection film 18 is formed.
The bonding pad 17 is usually formed on the transparent electrode layer 15, but may be directly formed on the upper contact layer 14 after some of the transparent electrode layer 15 is eliminated. An n-type nitride semiconductor layer of a high concentration or a superlattice layer made of the nitride semiconductor can be interposed between the upper contact layer 14 and the transparent electrode layer 15 to form a tunnel junction therebetween. The insulating substrate 10 is formed using sapphire, SiC, GaN, AIN or the like.
In order to fabricate such a LED, growth of single crystal is needed. A metal organic chemical vapor deposition (MOCVD) method is usually used. In this case, ammonia (NH3) is used as a supply source of nitrogen (N) for growing GaN. In order to grow GaN, H2 is usually used as a carrier gas. In order to grow InGaN, N2 is usually used as a carrier gas. Ammonia (NH3) is thermally stabilized and only several % of NH3 is decomposed at a temperature of over 1000° C. and contributes to the growth of GaN as a nitrogen (N) supply source. Accordingly, in order to increase efficiency of thermal decomposition, high temperature growth is inevitably needed. Also, very high NH3/Ga ratio is required to obtain GaN having good crystallization property.
Such a quite amount of NH3 generates a large amount of hydrogen as a byproduct. In this case, when p-type GaN is grown, hydrogen is combined with magnesium which acts as a p-type dopant, resulting in a bonding of magnesium (Mg)-hydrogen (H) atomics. Thus, magnesium (Mg) does not produce holes and the p-type GaN does not have the property of a semiconductor.
Therefore, after the p-type GaN is grown, it is subjected to a subsequent annealing process for breaking the magnesium(Mg)-hydrogen (H) atomic bonding at a temperature of 400° C. or more. For this reason, Mg provides holes and the p-type GaN has the property of a semiconductor.
As described above, the conventional nitride semiconductor light emitting device requires irradiation of electron beam or a process for annealing at a high temperature of over 400° C. in order to obtain p-type GaN of a high quality. However, this makes the process complicated and the active layer 13 can be thermally damaged during the annealing process. Resultantly, there is a high possibility of degrading the performance of the device.